In general, it is essential in the writing of data into an optical recording medium such as a compact disc or the like, or in the reading of the data written therein, that the focal position of the reader/writer head is appropriately and accurately on a data record surface of the optical recording medium. For this purpose, a possible deviation between the focal position of the reader/writer head and the data record surface of the optical recording medium is detected and the position of an optical system, such as a lens, is adjusted in the optical axis direction by an actuator by a distance corresponding to the deviation, to change the focal length of the optical system, so as to automatically attain the desired focalization.
A possible relative deviation of the focal position of the reader/writer head from the data record surface of the optical recording medium may be detected, for example, by the astigmatism method, the critical-angle method or the knife-edge method. The relative deviation of the optical recording medium from the focal position of the reader/writer head is detected as a deviation output after the photoelectric outputs from the photodetectors for detecting the focal position have been processed according to any of the methods as mentioned above. The relationship between the deviation of the optical recording medium from the forcal position and the deviation output based thereon is represented by an S curve in FIG. 2. In FIG. 2, a portion 1 around a zero-crossing point represents a focussed or focalized position. Whereas, portions 2 and 3 represent deviation outputs when the focus of the reader/writer head is deviated from the position to be focused or focalized.
The reading/writing operation is carried out at the position 1 of the above-mentioned S curve. When the reader/writer head is out of the position 1, a feedback control is applied to an actuator for moving a lens so that the outputs from the photodetectors may always be kept zero. Thus, the focal point of the reader/writer head is automatically maintained on the position of the data record surface of the optical recording medium to be focussed or focalized. This enables a stable read/write operation.
The conventional focussing position detecting methods as described above, however, involve the following problems.
The deviation output should ideally be zero, at the focussed or focalized position of the chrracteristic S curve a shown in FIG. 2, after the photoelectric outputs from the photodetectors have been processed. In effect, however, the deviation output will not be zero. More particularly, there is generally some offset in the differential amplifier due to possible variations in characteristics between the photodetectors and possible inaccuracy in the mounting positions of the photodetectors. Due to this offset in the differential amplifier, some voltage is always output. To solve this problem, the automatic focussing control is carried out, heretofore, based on a detection signal which contains this voltage output due to the offset.
In this connection, however, it is to be noted that when an output from a light source is increased, the outputs from the two photodetectors will increase proportionally at a fixed rate, but the difference between the two outputs will not increase at a fixed rate. Therefore, when it is required to increase the output from the light source, for example, for carrying out the writing operation, the amplitude of the S curve is amplified as shown in FIG. 3. As a result of this, if there is an offset in the differential amplifier, the output at the focussed position (see 1 of FIG. 3) can not be zero. The amplitude of this output is also amplified, also amplifying the difference. The so amplified difference is then output to the actuator to operate the same to change its position inaccurately or wrongly to an extent not allowable, even when the lens of the optical system is at an appropriate position capable of attaining the focussing.
Thus, the conventional focussing position detection methods have such a problem that misoperation is possibly caused and accurate focussing is not assured when the utput from the light source varies or is varied.